This invention relates to a method and means for coating particulate material in a rotating drum, including the application of a coating solution and powder to hasten the build up and drying of coating layers on the particulate material.
It has been known for many years to coat particulate materials, such as seeds, pharmaceutical products, snack foods, and breakfast cereals. In the seed industry, generally a coating which increases the weight of the seed up to 25% is referred to as pelletizing. Pelletizing provides uniformity of size to the particles. Depending on the extent of coating applied, pelletizing may provide a spherical shape to the particles which are normally elongated or irregularly shaped.
There are three primary reasons for providing a coating to seeds. First, a protective coating is often applied to seeds so as to prevent dust-off of prior treatments that have been applied. For example, a film of biological material may be applied to the seed to increase the viability of the seed, and a protective coating is needed to prevent the film from being worn away from the seed during handling. The protective coating allows for a reduction in the amount of film treatment that needs to be applied, thus reducing costs to the seed company. The protective coating also increases the effectiveness of the film treatment by encapsulating the film on the seed. Also, the protective coating reduces environmental hazards of the film treatment.
Another purpose of providing a coating to seeds and small pharmaceutical products is to increase the size of the particle. Particularly for seeds which are too small to be mechanically planted, such an increase in size allows small seeds to be planted mechanically, rather than manually, thereby resulting in significant labor savings, both during the planting and by eliminating manual thinning. Mechanical planting also reduces the seed population required for planting, thereby providing further cost savings.
A third benefit provided by seed coatings is to increase the visibility of the seeds during planting. Therefore, more accurate and less wasteful planting results.
In prior art pelletizing operations, a binder material in a solution was applied by spray, and then a bulking agent, such as clay or wood fibers, was manually applied to the seeds. After the two-step binding and bulking operations were completed, the coated seeds were then taken to an oven to bake dry. Thus, the prior art multiple-step process was slow, time consuming, and labor intensive.
In the food industry, snack foods and cereals are often coated with seasoning, such as sugar or salt, to add flavoring. Such seasoning is normally applied as a solution, with the coated product then being dried. However, too much solution or slow drying can result in a saturated or soggy underlying food product, which is undesirable.
Therefore, a primary objective of the present invention is the provision of an improved method and means for coating particulate material, such as seeds, pharmaceuticals and food products.
A further objective of the present invention is the provision of a method and means for coating object using a plurality of spray nozzles and powder applicators in a rotating drum.
Another objective of the present invention is the provision of a method and means for coating particulate material, wherein the coating solution includes both a liquid and a powder, and is applied and dried in a rotating drum.
A further objective of the present invention is the provision of a method and means for coating particulate material wherein multiple layers of coatings can be applied in a continuous or batch process.
Still another objective of the present invention is the provision of a method and means for coating particulate material in a rotating drum having high airflow through the drum so as to maintain low drying temperatures.
A further objective of the present invention is the provision of a method and means for coating particles wherein a binder and a bulking agent are applied simultaneously in a solution, and a powder is used to hasten the coating build up and to hasten drying of the solution.
Yet another objective of the present invention is the provision of a method and means for coating particulate material in a rotating drum which is maintained at a negative pressure.
A further objective of the present invention is the provision of a means for coating particulate material which is economical to manufacture, and efficient and durable in use.
These and other objectives will become apparent from the following description of the invention.
A method and means is provided for coating particulate material. In a first embodiment using a continuous coating operation, the apparatus includes an elongated cylindrical perforated drum having an upper inlet end and a lower outlet end for flow through of the particulate material. In a second embodiment using a batch coating operation, the perforated drum has one open end for loading and unloading particulate material. In both the continuous and batch operations, the drum is mounted on a frame within a housing, with the angle of tilt of the drum being adjustable. The drum is operatively connected to a motor for rotating the drum about the tilted axis. A bar having a plurality of spray nozzles and a plurality of pneumatic powder pumps mounted thereon extends into the drum. A liquid coating solution is sprayed from the nozzles as the drum rotates, thereby coating the particulate material. With multiple nozzles, one or more liquid coating solutions may be sprayed from the nozzles so as to coat the particulate material as the drum rotates. The powder pumps dispense powder from a powder feed tube onto the material as the drum rotates. The pumps are individually regulated so as to produce a uniform powder coating throughout the length of the drum. The powder is applied to the particulate material while the solution is being sprayed. The powder permits a quicker build-up of coating layers, and quicker drying of the solution, since the powder absorbs the liquid solution. Heated air is supplied to the drum so as to dry the coating layers onto the material substantially instantaneously. Electrical circuitry and a computer or micro-processor are provided for controlling the operation of the apparatus.
In the continuous coating operation, as the material is tumbled in the rotating drum and coated with the liquid solution and powder, it increases in size and weight. Since the drum is tilted, the material flows by gravity downwardly through the drum for eventual discharge through the outlet end. As the material progresses through the drum, it is coated with solution from the successive spray nozzles and with the powder from the successive pumps.
In the batch operation, as the material is coated with the liquid and powder, it increases in size and weight. Since the drum is tilted, the material accumulates adjacent the end wall and along the sidewall of the drum. As the drum rotates in a clockwise position, the material is tumbled in the six o""clock-nine o""clock region of the drum and coated with solution from the spray nozzles and powder from the pumps. Initially, the material forms a dense mass, and as the material is sprayed and increases in size, the larger particles migrate away from the end wall and cannot penetrate the mass of smaller particles adjacent the end wall. Eventually, substantially all of the material is uniformly coated, such that the material forms a new mass wherein the particles are slightly larger than the original mass formed by the uncoated particles. The process repeats itself, such that the particles are coated with additional solution from the spray nozzles and with the powder from the pumps, thereby again increasing in size and weight and migrating away from the end wall. This cycle continues until the particles achieve a desired uniform size.